Signal generation apparatus, level correction value calculation system, and level correction value calculation method

ABSTRACT

A signal generation unit 2, a DA converter 3, variable attenuators 40, 42, 44, and 46 that attenuate an analog signal converted by the DA converter 3, a measurement unit 6 that detects a level of the signal attenuated by the variable attenuators 40, 42, 44, and 46 and passed through one or more semiconductor components, and a control unit 7 that obtains a value of a step error, which is a correction value of an attenuation amount of the variable attenuators 40, 42, 44, and 46 in each of a plurality of steps obtained by dividing a maximum value of the attenuation amount of the variable attenuators 40, 42, 44, and 46 by a variation amount, which is a predetermined attenuation amount are included.

TECHNICAL FIELD

The present invention relates to a signal generation apparatus that generates an analog signal in a desired frequency band.

BACKGROUND ART

As a signal generation apparatus that generates an analog signal in a desired frequency band, for example, a signal generation apparatus that converts a digital signal generated by a signal generator into an analog signal with a DA converter, and generates the analog signal in the desired frequency band is known.

Such a signal generation apparatus is incorporated in, for example, a mobile terminal test apparatus or the like that tests a mobile communication terminal, and generates and outputs a test signal.

Patent Document 1 describes a signal generation apparatus including quadrature modulation means for modulating a local signal according to an output signal of a baseband signal generation unit, and a variable ATT for attenuating a level of a signal from the quadrature modulation means in steps, in which a first correction value for correcting a frequency response characteristic to an input of the variable ATT from the quadrature modulation means and a second correction value for correcting an attenuation amount error at each step of the variable ATT are stored, and correction is performed by varying a size of an output signal with respect to a baseband signal generation unit based on the first correction value and the second correction value corresponding to a frequency during a modulation signal output.

RELATED ART DOCUMENT Patent Document

-   [Patent Document 1] JP-A-2008-205812

DISCLOSURE OF THE INVENTION Problem that the Invention is to Solve

In this manner, in a case of obtaining a correction value of an attenuation amount at each step of a variable attenuator, measurement is performed by changing a gain of the signal generation unit and the attenuation amount of the variable attenuator. Meanwhile, if the gain of the signal generation unit is greatly changed, non-linearity of a semiconductor component on a path from the signal generation unit to a measurement unit affects, and the correction value with high accuracy cannot be obtained.

Accordingly, an object of the present invention is to provide a signal generation apparatus capable of obtaining a correction value for a variable attenuator with high accuracy by suppressing an influence of non-linearity of a semiconductor component on a path from a signal generation unit to a measurement unit.

Means for Solving the Problem

According to the present invention, there is provided a signal generation apparatus including: a signal generation unit that generates a predetermined digital signal; a DA converter that converts the digital signal generated by the signal generation unit into an analog signal of a predetermined radio frequency; a variable attenuator that attenuates the analog signal converted by the DA converter; a measurement unit that detects a level of the signal attenuated by the variable attenuator and passed through one or more semiconductor components; and a control unit that obtains a value of a step error, which is a correction value of an attenuation amount of the variable attenuator in each of a plurality of steps obtained by dividing a maximum value of the attenuation amount of the variable attenuator by a variation amount, which is a predetermined attenuation amount, by sequentially performing a step error initial value calculation process and a step error repetition calculation process, in which the step error initial value calculation process is a process of calculating a measurement value of the measurement unit when a gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to zero, as the value of the step error at zero attenuation amount, and the step error repetition calculation process is a process of setting a measurement value of the measurement unit when the gain of the signal generation unit is set to a negative variation amount and the attenuation amount of the variable attenuator is set to the attenuation amount at a time of the previous measurement, as a reference value, setting a value obtained by subtracting a measurement value of the measurement unit when the gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to an attenuation amount obtained by adding the variation amount to the attenuation amount at the time of the previous measurement from the reference value, as an error value, setting a value obtained by adding the error value and the value of the step error of the previous step, as the value of the step error of the next step, and repeating these processes.

With this configuration, the correction value of the variable attenuator with high accuracy can be obtained by calculating the correction value of the variable attenuator for each predetermined variation amount and suppressing an influence of non-linearity of a semiconductor component on a path from the signal generation unit to the measurement unit.

In addition, according to the present invention, there is provided a level correction value calculation system including: a signal generation apparatus including a signal generation unit that generates a predetermined digital signal, a DA converter that converts the digital signal generated by the signal generation unit into an analog signal of a predetermined radio frequency, a variable attenuator that attenuates the analog signal converted by the DA converter, and a control unit that obtains a value of a step error, which is a correction value of an attenuation amount of the variable attenuator in each of a plurality of steps obtained by dividing a maximum value of the attenuation amount of the variable attenuator by a variation amount, which is a predetermined attenuation amount, by sequentially performing a step error initial value calculation process and a step error repetition calculation process; and a measurement apparatus that detects a level of the signal output from the signal generation apparatus and transmits the level to the control unit, in which the step error initial value calculation process is a process of calculating a measurement value of the measurement apparatus when a gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to zero, as the value of the step error at zero attenuation amount, and the step error repetition calculation process is a process of setting a measurement value of the measurement apparatus when the gain of the signal generation unit is set to a negative variation amount and the attenuation amount of the variable attenuator is set to the attenuation amount at a time of the previous measurement, as a reference value, setting a value obtained by subtracting a measurement value of the measurement apparatus when the gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to an attenuation amount obtained by adding the variation amount to the attenuation amount at the time of the previous measurement from the reference value, as an error value, setting a value obtained by adding the error value and the value of the step error of the previous step, as the value of the step error of the next step, and repeating these processes.

With this configuration, the correction value of the variable attenuator with high accuracy can be obtained by calculating the correction value of the variable attenuator for each predetermined variation amount and suppressing an influence of non-linearity of a semiconductor component on a path from the signal generation unit to the measurement apparatus.

In addition, according to the present invention, there is provided a level correction value calculation method of a signal generation apparatus including a signal generation unit that generates a predetermined digital signal, a DA converter that converts the digital signal generated by the signal generation unit into an analog signal of a predetermined radio frequency, a variable attenuator that attenuates the analog signal converted by the DA converter, a measurement unit that detects a level of the signal attenuated by the variable attenuator and passed through one or more semiconductor components, and a control unit that obtains a value of a step error, which is a correction value of an attenuation amount of the variable attenuator in each of a plurality of steps obtained by dividing a maximum value of the attenuation amount of the variable attenuator by a variation amount, which is a predetermined attenuation amount, by sequentially performing a step error initial value calculation step and a step error repetition calculation step, in which the step error initial value calculation step is a step of setting a measurement value of the measurement unit when a gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to zero, as the value of the step error at zero attenuation amount, and the step error repetition calculation step is a step of setting a measurement value of the measurement unit when the gain of the signal generation unit is set to a negative variation amount and the attenuation amount of the variable attenuator is set to the attenuation amount at a time of the previous measurement, as a reference value, setting a value obtained by subtracting a measurement value of the measurement unit when the gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to an attenuation amount obtained by adding the variation amount to the attenuation amount at the time of the previous measurement from the reference value, as an error value, setting a value obtained by adding the error value and the value of the step error of the previous step, as the value of the step error of the next step, and repeating these steps.

With this configuration, the correction value of the variable attenuator with high accuracy can be obtained by calculating the correction value of the variable attenuator for each predetermined variation amount and suppressing an influence of non-linearity of a semiconductor component on a path from the signal generation unit to the measurement apparatus.

Advantage of the Invention

The present invention can provide a signal generation apparatus capable of obtaining a correction value of a variable attenuator with high accuracy by suppressing an influence of non-linearity of a semiconductor component on a path from a signal generation unit to a measurement unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a signal generation apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram for explaining a method of calculating a step error value of the signal generation apparatus according to the embodiment of the present invention.

FIG. 3 is a flowchart explaining a procedure of a step error calculation process of the signal generation apparatus according to the embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a signal generation apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.

In FIG. 1 , a signal generation apparatus 1 according to the embodiment of the present invention is configured with a signal generation unit 2, a DA converter (digital to analog converter (DAC)) 3, an RF module 4, an AD converter (analog to digital converter (ADC)) 5, a measurement unit 6, and a control unit 7.

The signal generation unit 2 generates and outputs a predetermined digital signal. The signal generation unit 2 can adjust a digital gain of the output digital signal. The digital gain of the digital signal output from the signal generation unit 2 is adjusted under control of the control unit 7.

The DA converter 3 converts the digital signal generated by the signal generation unit 2 into an analog signal of a predetermined radio frequency.

The RF module 4 adjusts a level of the analog signal output from the DA converter 3, and outputs the analog signal from the output terminal 10.

The RF module 4 is configured with variable attenuators 40, 42, 44, and 46 that adjust a level of an input signal, amplifiers 41, 43, 45, and 49 that amplify the input signal, a divider 47 that distributes an analog signal output by a variable attenuator 46 to the output terminal 10 or a switch 48, and the switch 48 that selects any of the analog signal output by the divider 47 or the analog signal input to an input terminal 11 and output the signal to the amplifier 49.

The variable attenuators 40, 42, 44, and 46 change the attenuation amount under the control of the control unit 7. The switch 48 switches a signal to be output to the amplifier 49 under the control of the control unit 7.

The AD converter 5 converts the analog signal input to the input terminal 11 or the analog signal returned within the RF module 4 into a digital signal.

The measurement unit 6 measures a level and the like of the digital signal output from the AD converter 5. The measurement unit 6 performs measurement under the control of the control unit 7, and transmits the measurement result to the control unit 7.

The control unit 7 is configured by a computer apparatus, for example. This computer apparatus includes a central processing unit (CPU), a read only memory (ROM), a random access memory (RAM), a non-volatile storage medium such as a hard disk device, various types of input and output ports, a display device, an input device such as a pointing device or a keyboard device (each is not illustrated).

A program for causing the computer apparatus to function as the control unit 7 is stored in the ROM and the hard disk device of the computer apparatus. That is, the computer apparatus functions as the control unit 7 by the CPU executing the program stored in the ROM by using the RAM as a work region.

The signal generation apparatus 1 can select an External path for adjusting a level of a signal generated by the signal generation unit 2 in the RF module 4 and outputting the signal from the output terminal 10, indicated by a dotted line A in FIG. 1 , and an Internal path for inputting a signal generated by the signal generation unit 2 and level-adjusted by the RF module 4 to the measurement unit 6 via the AD converter 5 after being returned by the RF module 4, indicated by a dotted line B in FIG. 1 .

The variable attenuators 40, 42, 44, and 46 of the present embodiment have non-linear characteristics. It is necessary to correct this non-linearity, but if a gain of the signal generation unit 2 is greatly changed, the non-linearity of a semiconductor component on the path from the signal generation unit 2 to the measurement unit 6 will affect, the correction value with high accuracy cannot be obtained.

Therefore, in the present embodiment, the maximum value of the attenuation amount of the variable attenuators 40, 42, 44, and 46 is divided by a variation amount, which is a predetermined attenuation amount, and a step error value is calculated as an error when the attenuation amount is changed stepwise by the variation amount, and the attenuation amount is corrected.

For example, in a case where a settable attenuation amount of the variable attenuators 40, 42, 44, and 46 is zero to 30 dB and the minimum resolution is 1 dB, an error when the attenuation amount of the variable attenuators 40, 42, 44, and 46 is divided by 1 dB and the attenuation amount is changed stepwise by 1 dB is calculated as the step error value.

The control unit 7 sets an Internal path for returning a signal generated by the signal generation unit 2 and level-adjusted by the RF module 4 in the RF module 4, and changes a gain of the signal generated by the signal generation unit 2 and the attenuation amount of the variable attenuators 40, 42, 44, and 46 to calculate a step error value.

The control unit 7 sets a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to zero and the attenuation amount of the variable attenuators 40, 42, 44, and 46 is set to zero, as the step error value at zero attenuation amount (step error initial value calculation process).

For example, the control unit 7 sets a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to a negative variation amount and the attenuation amount of the variable attenuators 40, 42, 44, and 46 is set to the attenuation amount at a time of the previous measurement, as a reference value.

The control unit 7 sets a value obtained by subtracting a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to zero and the attenuation amount of the variable attenuators 40, 42, 44, and 46 is set to an attenuation amount obtained by adding the variation amount to the attenuation amount at a time of the previous measurement from the reference value, as an error value.

The control unit 7 sets a value obtained by adding the error value and the step error value of the previous step, as a step error value of the next step.

Next, the control unit 7 sets a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to a negative variation amount and the attenuation amount of the variable attenuators 40, 42, 44, and 46 is set to the attenuation amount at a time of the previous measurement as the reference value, and repeats these processes to calculate the step error value at each step (step error repetition calculation process).

FIG. 2 illustrates a calculation method in which in a case where a settable attenuation amount of the variable attenuators 40, 42, 44, and 46 is zero to 30 dB and the minimum resolution is 1 dB, a step error value when the attenuation amount of the variable attenuators 40, 42, 44, and 46 is divided by 1 dB and the attenuation amount is changed stepwise by 1 dB is calculated. The step error value is calculated for each of the variable attenuators 40, 42, 44, and 46.

In FIG. 2 , the control unit 7 sets a measurement value of the measurement unit 6 when a gain of the signal generation unit 2 is set to zero and the attenuation amount of the variable attenuators 40, 42, 44, and 46 is set to zero, as the step error value at zero attenuation amount.

The control unit 7 sets a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to −1 dB and the attenuation amount of the target variable attenuator is set to zero, as the reference value.

The control unit 7 causes the measurement unit 6 to measure a measurement value when the gain of the signal generation unit 2 is set to zero and the attenuation amount of a target variable attenuator is set to 1 dB, which is obtained by adding the variation amount to the attenuation amount at a time of the previous measurement.

The control unit 7 set a value obtained by adding the step error value at zero attenuation amount and a value obtained by subtracting the current measurement value from the reference value to a step error value at the attenuation amount of 1 dB, which is the next step.

The control unit 7 sets a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to −1 dB and the attenuation amount of a target variable attenuator is set to 1 dB of the attenuation amount at a time of the previous measurement, as a reference value.

The control unit 7 causes the measurement unit 6 to measure a measurement value when the gain of the signal generation unit 2 is set to zero and the attenuation amount of a target variable attenuator is set to 2 dB, which is obtained by adding the variation amount to the attenuation amount at a time of the previous measurement.

The control unit 7 set a value obtained by adding the value of the step error at the attenuation amount of 1 dB of the previous step and a value obtained by subtracting the current measurement value from the reference value to a step error value at 2 dB attenuation amount, which is the next step.

The control unit 7 sets a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to −1 dB and the attenuation amount of a target variable attenuator is set to 2 dB, which is the attenuation amount at a time of the previous measurement, as a reference value.

The control unit 7 causes the measurement unit 6 to measure a measurement value when the gain of the signal generation unit 2 is set to zero and the attenuation amount of a target variable attenuator is set to 3 dB, which is obtained by adding the variation amount to the attenuation amount at a time of the previous measurement.

The control unit 7 set a value obtained by adding the value of the step error at the attenuation amount of 2 dB of the previous step and a value obtained by subtracting the current measurement value from the reference value to a step error value at 3 dB attenuation amount, which is the next step.

The control unit 7 repeats such a process to set a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to −1 dB and the attenuation amount of a target variable attenuator is set to 29 dB, which is the attenuation amount at a time of the previous measurement, as a reference value.

The control unit 7 causes the measurement unit 6 to measure a measurement value when the gain of the signal generation unit 2 is set to zero and the attenuation amount of a target variable attenuator is set to 30 dB, which is obtained by adding the variation amount to the attenuation amount at a time of the previous measurement.

The control unit 7 set a value obtained by adding the step error value at the attenuation amount of 29 dB of the previous step and a value obtained by subtracting the current measurement value from the reference value to a step error value at 30 dB attenuation amount, which is the next step, to calculate the step error value up to 30 dB.

The step error calculation process by the signal generation apparatus 1 according to the present embodiment configured as described above will be described with reference to FIG. 3 . The step error calculation process described below is started when step error calculation is selected by an operation on an input device of the control unit 7.

In step S1, the control unit 7 sets a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to zero and the attenuation amount of the variable attenuators 40, 42, 44, and 46 is set to zero, as a step error value at zero attenuation amount (step error initial value calculation step). After executing the process in step S1, the control unit 7 executes the process in step S2.

In step S2, the control unit 7 sets a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to a negative variation amount and the attenuation amount of the variable attenuators 40, 42, 44, and 46 is set to the attenuation amount at a time of the previous measurement, as a reference value. After executing the process in step S2, the control unit 7 executes the process in step S3.

In step S3, the control unit 7 sets a value obtained by subtracting a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to zero and the attenuation amount of the variable attenuators 40, 42, 44, and 46 is set to an attenuation amount obtained by adding the variation amount to the attenuation amount at a time of the previous measurement from the reference value, as an error value. After executing the process in step S3, the control unit 7 executes the process in step S4.

In step S4, the control unit 7 sets a value obtained by adding the error value and the step error value of the previous step, as the step error value of the next step. After executing the process in step S4, the control unit 7 executes the process in step S5.

In step S5, the control unit 7 determines whether or not the calculation of the step error value is executed up to an upper limit attenuation amount of the variable attenuators 40, 42, 44, and 46.

In a case where it is determined that the calculation of the step error value is executed up to the upper limit attenuation amount, the control unit 7 ends the step error calculation process. In a case where it is determined that the step error value is not calculated up to the upper limit attenuation amount, the control unit 7 executes the process in step S2. Steps S2 to S5 are defined as a step error repetition calculation step.

In this manner, in the embodiment described above, the control unit 7 divides the attenuation amount of the variable attenuators 40, 42, 44, and 46 by the variation amount, which is a predetermined attenuation amount, sets a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to zero and the attenuation amount of the variable attenuators 40, 42, 44, and 46 is set to zero as the step error value at zero attenuation amount, sets a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to the negative variation amount and the attenuation amount of the variable attenuators 40, 42, 44, and 46 is the attenuation amount at a time of the previous measurement as the reference value, sets a value of subtracting a measurement value of the measurement unit 6 when the gain of the signal generation unit 2 is set to zero and the attenuation amount of the variable attenuators 40, 42, 44, and 46 is set to an attenuation amount obtained by adding the variation amount to the attenuation amount at the time of the previous measurement from the reference value as the error value, sets a value obtained by adding the error value and the step error value of the previous step as the step error value of the next step, and repeats these processes.

Therefore, the correction value of the variable attenuators 40, 42, 44, and 46 with high accuracy can be obtained by calculating the correction value of the variable attenuators 40, 42, 44, and 46 for each predetermined variation amount and suppressing an influence of non-linearity of a semiconductor component on a path from the signal generation unit 2 to the measurement unit 6.

In the present embodiment, the Internal path is set, the signal is returned inside the signal generation apparatus 1, and the measurement is performed by the measurement unit 6. Meanwhile, the External path may be set and an external measurement apparatus may be connected to the output terminal 10, and the system may be provided such that a measurement value is transmitted from the measurement apparatus to the control unit 7.

Although the embodiment of the present invention is disclosed, it is apparent that the embodiment can be modified by those skilled in the art without departing from the scope of the present invention. All such modifications and equivalents are intended to be included in the following claims.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   -   1: Signal generation apparatus     -   2: Signal generation unit     -   3: DA converter     -   4: RF module     -   5: AD converter     -   6: Measurement unit     -   7: Control unit     -   40, 42, 44, 46: Variable attenuator 

What is claimed is:
 1. A signal generation apparatus comprising: a signal generation unit that generates a predetermined digital signal; a DA converter that converts the digital signal generated by the signal generation unit into an analog signal of a predetermined radio frequency; a variable attenuator that attenuates the analog signal converted by the DA converter; a measurement unit that detects a level of the signal attenuated by the variable attenuator and passed through one or more semiconductor components; and a control unit that obtains a value of a step error, which is a correction value of an attenuation amount of the variable attenuator in each of a plurality of steps obtained by dividing a maximum value of the attenuation amount of the variable attenuator by a variation amount, which is a predetermined attenuation amount, by sequentially performing a step error initial value calculation process and a step error repetition calculation process, wherein the step error initial value calculation process is a process of calculating a measurement value of the measurement unit when a gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to zero, as the value of the step error at zero attenuation amount, and the step error repetition calculation process is a process of setting a measurement value of the measurement unit when the gain of the signal generation unit is set to a negative variation amount and the attenuation amount of the variable attenuator is set to the attenuation amount at a time of the previous measurement, as a reference value, setting a value obtained by subtracting a measurement value of the measurement unit when the gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to an attenuation amount obtained by adding the variation amount to the attenuation amount at the time of the previous measurement from the reference value, as an error value, setting a value obtained by adding the error value and the value of the step error of the previous step, as the value of the step error of the next step, and repeating these processes.
 2. A level correction value calculation system comprising: a signal generation apparatus including a signal generation unit that generates a predetermined digital signal, a DA converter that converts the digital signal generated by the signal generation unit into an analog signal of a predetermined radio frequency, a variable attenuator that attenuates the analog signal converted by the DA converter, and a control unit that obtains a value of a step error, which is a correction value of an attenuation amount of the variable attenuator in each of a plurality of steps obtained by dividing a maximum value of the attenuation amount of the variable attenuator by a variation amount, which is a predetermined attenuation amount, by sequentially performing a step error initial value calculation process and a step error repetition calculation process; and a measurement apparatus that detects a level of the signal output from the signal generation apparatus and transmits the level to the control unit, wherein the step error initial value calculation process is a process of calculating a measurement value of the measurement apparatus when a gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to zero, as the value of the step error at zero attenuation amount, and the step error repetition calculation process is a process of setting a measurement value of the measurement apparatus when the gain of the signal generation unit is set to a negative variation amount and the attenuation amount of the variable attenuator is set to the attenuation amount at a time of the previous measurement, as a reference value, setting a value obtained by subtracting a measurement value of the measurement apparatus when the gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to an attenuation amount obtained by adding the variation amount to the attenuation amount at the time of the previous measurement from the reference value, as an error value, setting a value obtained by adding the error value and the value of the step error of the previous step, as the value of the step error of the next step, and repeating these processes.
 3. A level correction value calculation method of a signal generation apparatus including a signal generation unit that generates a predetermined digital signal, a DA converter that converts the digital signal generated by the signal generation unit into an analog signal of a predetermined radio frequency, a variable attenuator that attenuates the analog signal converted by the DA converter, a measurement unit that detects a level of the signal attenuated by the variable attenuator and passed through one or more semiconductor components, and a control unit that obtains a value of a step error, which is a correction value of an attenuation amount of the variable attenuator in each of a plurality of steps obtained by dividing a maximum value of the attenuation amount of the variable attenuator by a variation amount, which is a predetermined attenuation amount, by sequentially performing a step error initial value calculation step and a step error repetition calculation step, wherein the step error initial value calculation step is a step of setting a measurement value of the measurement unit when a gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to zero, as the value of the step error at zero attenuation amount, and the step error repetition calculation step is a step of setting a measurement value of the measurement unit when the gain of the signal generation unit is set to a negative variation amount and the attenuation amount of the variable attenuator is set to the attenuation amount at a time of the previous measurement, as a reference value, setting a value obtained by subtracting a measurement value of the measurement unit when the gain of the signal generation unit is set to zero and the attenuation amount of the variable attenuator is set to an attenuation amount obtained by adding the variation amount to the attenuation amount at the time of the previous measurement from the reference value, as an error value, setting a value obtained by adding the error value and the value of the step error of the previous step, as the value of the step error of the next step, and repeating these steps. 